(1) Are there any living organisms that might be helpful in visualizing "transitional forms" between unicellular organisms and multicellular animals? E.g. I have heard of slime molds as one example. Would love to hear of as many good examples as you can come up with. Not looking for strict hereditary relationships here, and I am fine with plausible but unproven theories. E.g., mudskippers might help a person visualize the evolution of amphibians, even if they aren't in fact closely related to amphibian ancestors.

(2) Do we think that the first animals evolved into sponges, or cnidarians/ctenophoroans, or something else? Did sponges and cnidarians/ctenophoroans evolve multicellularity separately? As this is probably a big topic, feel free to just post a link or two.

(3) For any examples that you can think of for #1, would love to hear of any good citations of articles or books for me to read, but keep in mind that I am not a scientist and not looking for anything extremely technical.

$\begingroup$Welcome to Biology.SE. I am afraid your question is way too broad to be answered here. A SE post must always be restricted to a single, clearly defined question that can ideally be answered without having to write a whole book!$\endgroup$
– Remi.bJun 27 '18 at 22:27

$\begingroup$Thanks, that article looks like what I needed, and contains a few details I hadn't thought about -- e.g., the development of more than one cell type within an organism that has unicellular living relatives.$\endgroup$
– Greg BoettcherJun 29 '18 at 10:57

There is no agreed scenario of evolution of animals. Molecular evidence is ambiguous, early fossils are puzzling in form, and even more in relations.

Molecular phylogenies agree that closest unicellular relatives of animals are choanoflagellates. Most (nearly all) researchers interpret it that our distant ancestor was choanoflagellate-like organism. However there are people that think that choanoflagellates are sponges that "returned" to unicellular life.

Do we think that the first animals evolved into sponges, or cnidarians/ctenophoroans, or something else?

Molecular phylogenies of animals are not clear. I write below few examples of most basal taxa from different articles, all based on molecular evidence. Most basal is on the left.

Porifera are sponges, I marked with porifera* paraphyletic reconstruction, and with porifera monophyletic. Groups homoscleromorpha, and demospongia are subgroups of sponges. As we see the most basal are either sponges or ctenophora (comb jellies), after them usually placozoa are second. Cnidaria with bilateria are least basal.

Most animals except most sponges and placozoa have basement membrane. It is a non cellular collagen matrix under virtually all epithelial tissues (tissue organized as 2d sheet of cells). In sponges only one group, called homoscleromorpha in larval stage was reported to have this membrane. This would suggest that all other animals derive from that group. However other source contradicts that finding: https://www.elsevier.com/books/book-series/advances-in-marine-biology vol 61.

Fossil evidence of most basal animal is even more problematic. There are plenty of early sponge fossils, some of them going even before Marinoan glaciation (about 90My before Cambrian), check this one, however none of the Precambrian fossils are uncontested. This is especially significant as sponge fossils with their robust spicula, and macroscopic sizes, are usually quite well-preserved fossils. Moreover, there are great richness of soft-bodied animals preserved from Ediacaran, but sponges are missing. This suggests that sponges before Cambrian would have to be very small, and without spicula, what is possible, however not very likely.

If precambrian sponges are rejected then sponge-like animals could not be basal animals. Scenario consistent with sponges being most basal is that they either evolved multicellularity independently in Cambrian, or that they evolved in Cambrian from animals of different form that separated from our lineage in Precambrian.

If we reject sponges, then most basal living animals are either placozoa or ctenophora. Placozoa taxon contains just one genus Trichoplax with few species of a very weird animal. Ctenophora (comb-jellies) is a small group with about 100 species of slowly moving marine predators.

Earliest unquestionably animal fossils are from Avalon assemblage from Ediacaran period (about 30My before Cambrian). Figure 1 from here is quite telling. Those earliest fossils seem to be sessile growing on surface of bacterial mat, bound to the ground with gelatin-like substance produced by bacteria. They were likely filter-feeders, possibly with symbiotic algae, just like modern sponges. However their form is very different from sponges; in shape they are more similar to plants or fractals. You can read about those sessile forms here, and here. Later mobile animals evolved, most iconic of them Dickinsonia. Here is an album of many of Ediacaran forms, that could be a good starting point. First mobile animals like Dickinsonia, or Yorgia were probably feeding on biomats. They didn't have mouth, probably digesting bacteria externally under bottom of their body. Only extant animal that feeds that way is placozoan Trichoplax. There is possibility that they are related.

There are plenty of sources to learn about Ediacaran biota, a lot is known about them. However question which one of them, if any, was the most basal animal was not answered till now. In other words you can read a lot about those fossils, but still you will not learn how the first animal looked.

For any examples that you can think of for living organisms that might be helpful in visualizing "transitional forms"

Though probably not the most basal animal, it will be helpful for you to read about Trichoplax. I suggest this, especially figure 7 is nice ilustration.

$\begingroup$Thank you! I had heard that choanoflagellates were likely related to sponge ancestors, and it intrigues me that free-living unicellular organisms might go from using their flagella to swim around, to using their flagella to draw a current of nutrient-rich water through the body of a sponge. Truth be told, I am a fiction writer and was hoping for more details of this sort to inspire me to write a story about the evolution of multicellular animals, even if the story is not documentably accurate in all its details. I was hoping for more examples, but thanks, this is a good one.$\endgroup$
– Greg BoettcherJun 29 '18 at 10:34

$\begingroup$One thing you could think about it how evolutionarily difficult (and thus rare) multicellularity is, because evolution favors selfishness. For a colony of individual cells, each independently reproducing, to become a multicellular organism, then some of those cells must give up their own reproduction to support others. This is "against the rules".Choanoflagellates do congregate, but remain reproductively independent.$\endgroup$
– Karl KjerJun 29 '18 at 13:27